1. Field of the Invention
Aspects of the present invention relate generally to an image forming apparatus, and more particularly, to a thermal transfer image forming apparatus with an improved structure which enables the apparatus to form high quality images.
2. Description of the Related Art
A thermal transfer image forming apparatus applies heat to an ink ribbon which contacts a print medium, such as a sheet of paper, a transparency sheet, etc., and transfers ink to the print medium, thereby forming an image. In general, the thermal transfer image forming apparatus includes a thermal head which selectively heats a plurality of heating elements to perform printing, and a platen roller which presses against the thermal head at a predetermined pressure.
This thermal transfer image forming apparatus forms high quality images only when the thermal head is pressed onto the platen roller at a uniform pressure. U.S. Patent Publication No. 2005/0024473 discloses a thermal transfer image forming apparatus which automatically adjusts the thermal head when the thermal head is not pressed onto the platen roller at a uniform pressure.
This conventional thermal transfer image forming apparatus includes a first head supporting member to support a thermal head and a second head supporting member to support two elastic members which elastically push the first head supporting member toward a platen roller. The first head supporting member includes first and second arcuate arm parts which extend from both longitudinal ends of the first head supporting member in one direction. The second head supporting member includes third and fourth arm parts which are disposed adjacent to the first and second arm parts, respectively. Specifically, one end of the first arm part is supported by one end of the third arm part such that the thermal head moves in a parallel direction to a direction in which the thermal head moves towards or away from the platen roller. The two elastic members disposed between the first and second head supporting members are designed to push the first head supporting member towards the platen roller with different elastic forces.
Therefore, when the thermal head is not in uniform contact with the platen roller due to dimensional errors of or assembly imbalance between the parts of the first and second head supporting members, the first arm part of the first head supporting member moves relative to the third arm part of the second head supporting member to automatically adjust the thermal head so that the thermal head makes uniform contact with the platen roller. Furthermore, the two elastic members elastically pushing the thermal head are designed to supply different elastic supporting forces to the thermal head, so that although the thermal head has an automatically adjustable structure, the pressure acting on both ends of the thermal head maintains uniformity.
However, in this conventional thermal transfer image forming apparatus, the one end of the first head supporting member which supports the thermal head moves freely. Hence, the alignment of the thermal head is affected by an external impact. Specifically, when the conventional thermal transfer image forming apparatus is impacted by even a slight external force, the alignment of the thermal head fluctuates. When the alignment of the thermal head fluctuates, the contact between the thermal head and the platen roller also fluctuates, and the pressure which the thermal head applies to the platen roller changes frequently.
Furthermore, in the conventional thermal transfer image forming apparatus, when the pressure acting on both ends of the thermal head changes due to external impacts or repeated operations, the print quality become poor. Also, the pressure cannot be easily adjusted in the conventional thermal transfer image forming apparatus.